Conventional optical coatings are designed primarily for optical performance. Thus, stress within the coatings is not controlled, or only crudely controlled, so that the coating does not flake off of the substrate to which it is applied. As a result, coatings that must be hardened for operation in adverse environments, such as for high power laser, x-ray, nuclear, high temperature or cryogenic, or high thermal flux applications; or those coatings required to be made very thick, such as rugate coatings, are prone to stress failures. To overcome this limitation, while producing coatings with suitable optical properties, a technique is required wherein both the stress and the coefficient of thermal expansion (CTE) of the coating is either made small or is controlled.
Reference is made to an article entitled "Stresses Developed in Optical Film Coatings", Applied Optics, Vol. 5, No. 1, January 1966, by Anthony E. Ennos. This article describes the use a thin silica strip as one mirror of a laser interferometer that is mounted within a coating chamber. As the coating is deposited upon the strip a bending of the strip occurs. The bending is measured by the interferometer and is correlated with stress in the coating.
Most conventional optical coatings are fabricated by employing a sequential and/or co-evaporation of two materials. The material and coating design are determined by the optical performance. Stress is considered only though the selection of material pairs that produce durable coatings. It has also been known that gradient index of refraction coatings and rugate coatings may be fabricated by the co-evaporation of two optical materials. For a gradient index coating the mixture of materials is chosen so as to control the index of refraction as a function of coating thickness. However, little or no consideration has been directed to the problem of stress buildup within conventional coatings.
As an example, FIG. 1 illustrates in graphical form the stress buildup within a multilayer coating due to layer stress mismatch. Ideally, the stress within the individual coating layers is alternately tensile and compressive. However, stress balance is never perfect and, thus, residual stress builds up within the coating as the thickness is increased.
As a result, at some thickness (T.sub.F), a stress-induced failure of the coating occurs. This stress induced failure adversely affects the optical properties and/or the physical properties of the coating.
Also by example, reference is made to FIG. 6 which shows the effect of a deposition of a ThF.sub.4 coating upon a Cer-Vit substrate (curve A) and the effect of the deposition of the ThF.sub.4 coating upon a KCl substrate (curve B). As can be seen, during deposition (region C) the film tensile stress for both substrate systems increases and then levels off, although each attains a different final tensile strength value. However, during cooling (region D) the film stress for the ThF.sub.4 /Cer-Vit system continues to increase in tensile stress whereas the ThF.sub.4 /KCl material system develops a compressive film stress. These changes in the stress during cooling result from the differential contraction between the coating and the substrate, in that the coating and substrate have different CTEs (Coefficients of Thermal Expansion). As can be appreciated, the resulting induced stress within each of these systems may prove detrimental during use of the deposited coatings, especially if such use tends to further increase the tensile stress for the ThF.sub.4 /Cer-Vit material system or to further increase the compressive stress for the ThF.sub.4 /KCl material system.
It is thus one object of the invention to provide coatings for optical devices wherein the stress and/or CTE of a coating is specified and controlled.
It is a further object of the invention to provide method and apparatus for fabricating a coating upon a substrate such that the optical properties and the stress and/or CTE properties of the coating are simultaneously controlled by the co-evaporation or sequential evaporation of a plurality of coating materials.
It is a still further object of the invention to provide method and apparatus for fabricating thick coatings, such as rugate coatings, that exhibit a higher optical density and/or a narrower bandwidth than attainable with conventional coating methodology, while controlling the stress and/or CTE characteristics of the coating to be within prescribed values.